Various circuits exist for determining which key is pressed on a keypad of a remote control device. One older circuit includes a voltage dividing network of resistors and an integrated circuit with an analog-to-digital converter (ADC). When a key on the keypad is pressed, a node in the voltage dividing network is connected to a ground lead such that the voltage dividing network outputs an encoding signal voltage that is unique for the specific key that is pressed. The voltage dividing network contains a string of discrete series-connected resistors. The ADC receives the encoding signal voltage and converts it into a digital value that corresponds to the pressed key. U.S. Pat. No. 4,015,254 describes a keyboard encoding circuit with a voltage dividing network.
Some keypad encoding applications are extremely cost sensitive. Each discrete component (for example, a discrete surface mount resistor) adds cost to the circuit. Keypad encoding circuits have been developed that achieve substantial cost savings by reducing the use of discrete components.
FIG. 1 (prior art) is a schematic diagram of one such circuit, called a keypad matrix scanning circuit 10. The keys (0-9, and POWER) of a keypad are arranged in a matrix. Vertical conductors connect keys oriented in columns, and horizontal conductors connect keys along rows. A landing pad is disposed at each location where a vertical conductor and horizontal conductor cross. For example, landing pad 11 is disposed at the intersection of a row conductor and a column conductor at the five key. When a key located at an intersection is pressed, a conductive portion of the key makes contact with both sides of the landing pad thereby coupling the vertical and horizontal conductors together. An integrated circuit 12 periodically scans all keys in the matrix by successively transmitting a digital code on each of the conductors in one dimension and then detecting which of the conductors in the other dimension returns the code.
To determine that a key has been pressed, for example the five key, integrated circuit 12 successively transmits an encoding signal voltage, such as a digital one, on each of its row terminals A, B, C and D. Integrated circuit 12 detects a digital one on a column terminal only at the instant when a digital one is transmitted from output terminal C. This digital one is detected only on column terminal F. The keypad encoding circuit 10 determines that the five key has been pressed because row terminal C and column terminal F can be coupled only by the five key.
In keypad matrix scanning circuit 10, integrated circuit 12 has a separate terminal for each row and column of the keypad matrix. Integrated circuit 12 has four row terminals A, B, C and D, three column terminals E, F, and G, one VCC power terminal and one ground terminal. This large number of terminals results in the package of the integrated circuit being more expensive due to having to provide a large number of terminals (i.e., pins) on the package.
Cost is a major consideration in the business of infrared transmitting remote control devices for electronic appliances. It is not uncommon for a remote control device to have more than thirty-six keys, for example. To scan thirty-six keys, an integrated circuit package having at least twelve terminals would be required. Each of these terminals adds substantial cost. A large number of terminals also often results in the integrated circuit die contained in the package being more expensive due to having to provide a large number of bonding pads and associated bond wires.
The cost of a remote control device can be reduced not only by reducing the number of discrete components in the keypad encoding circuit and the number of integrated circuit terminals, but also by reducing the number of layers on the printed circuit board upon which the keyboard matrix scanning circuitry is disposed. The horizontal and vertical conductors of the scanning circuitry can, for example, be realized on a two-sided printed circuit board having an etched conductive layer of traces on each side. The horizontal conductors may, for example, be traces disposed on one side of the printed circuit board whereas the vertical conductors are traces disposed on the other side. Metal landing pads are placed at the intersection of the column and row traces. To prevent the metal landing pads on the printed circuit board from oxidizing, becoming more resistive, and compromising operation of the keypad, the metal of the landing pads is covered with a conductive carbon layer. The conductive carbon layer is deposited using an inexpensive silk screen process. Providing each of the two layers of traces and the conductive carbon layer in this printed circuit board, however, involves cost.
A lower-cost keypad encoding circuit therefore involves a single-sided printed circuit board. The conductors in one dimension are formed as conventional etched metal traces (for example, copper traces) on the printed circuit board. A solder mask is then disposed over the copper traces. The conductors in the other dimension are then formed as traces of conductive carbon on top of the solder mask using a silk screen mask. Because these conductive carbon traces are formed of the same layer that is used to protect the landing pads from oxidation, providing the second layer of traces does not involve the additional cost associated with providing a second layer of etched traces in a conventional two-sided printed circuit board.
Although the cost of providing keypad encoding circuitry on a remote control device has been reduced considerably, additional cost savings are desired. In the specific business of television remote control devices, the cost savings achieved by eliminating even one discrete resistor, one integrated circuit terminal or one printed circuit board layer is significant. A less expensive keypad encoding circuit is sought.